The laser flash method is well established and widely used for thermal conductivity measurements. The method requires a disk-shaped specimen having a thickness greater than 1 millimeter and a diameter typically of 10 millimeters. The method can only measure the average value of the disk-shaped specimen and is unsuitable for measurements of thermal conductivity of regions or features that are on the scale of micrometers. Such micro-scale measurements are important for microelectronics industries with miniaturized devices and for high throughput screening of micro-scale combinatorial libraries.
The apparatus and method disclosed by Baba et al. in U.S. Pat. No. 6,595,685B2 allow micro-scale measurements of thermal properties to be made. They use two continuum wave lasers. One laser serves as the pump beam and is used to heat the surface of the specimen with a sinusoidally modulated intensity. The other laser serves as the probe beam that detects the reflectivity of the specimen surface as a measure of the surface temperature. The two beams are focused on the same spot. Baba et al. analyze the data with only a two-layer model and one-dimensional heat flow without taking into account the interface effects. The method may work only for low thermal conductivity materials when the lateral heat flow and the interface effects are small. But, even for the low conductivity glass, the method is not accurate as demonstrated by the data shown in Table I of the aforementioned US patent. The thermal effusivity of glass varies from 30% too high to 50% too low depending on the thickness of the molybdenum film deposited for the measurements. For high thermal conductivity materials such as metals, the short time thermal response (on the order of 100 picoseconds) is critical for accurate thermal property assessment, but that time scale is not accessible with continuum wave lasers. Therefore, accuracy of the data obtained from the aforementioned method is poor.
Improved measurement accuracy is achieved by methods disclosed by Baba in U.S. Pat. No. 6,592,252B2 and by Taketoshi and Baba in U.S. patent application number U.S. 2003/0202556A1. Both embodiments use rear heating-front probing configuration, and thus may only be applied to thin films deposited on optically transparent substrates. Furthermore, the metal films have to be thick enough to be optically opaque but not too thick to keep the thermal pulse from arriving within the time-window (a few nanoseconds) of the apparatus. Therefore, these two apparata put severe constraints on the geometries of the materials that are to be measured. Both embodiments use a pump beam to heat the specimen from one side of the thin film and a probe beam to detect the temperature change from the opposite side. The two-sided approach is simpler in mathematical equations to evaluate the thermal properties; however, the alignment of the beams for the two-sided measurement is laborious and extremely difficult.
These current apparata and methods have significant drawbacks: 1) the one-sided method has poor accuracy; 2) the two-sided methods put severe constraints on the specimen geometries; and, 3) all the aforementioned methods are suitable to only limited types of materials such as low conductivity materials. Thus, there is a need for apparata and methods that allow measurements of thermal conductivity to be made more reliably, accurately, conveniently and easily. There is also a need for such apparata and methods to allow accurate and convenient measurements to be made of a broad range of materials including metals, ceramics and polymers. There is yet a further need for such apparata and methods to be able to measure thermal properties in the micro-scale. There is still a further need for such apparata and methods to perform measurements on both bulk specimens and thin film specimens without laborious sample preparation or system alignment.